1. Field of the Invention
The present invention relates to a control device for an electrically driven variable valve timing apparatus, which uses an electric motor as its drive source.
2. Description of Related Art
Lately, an electric motor is often used as a drive source of a variable valve timing apparatus in place of a hydraulic system to implement a precise variable valve timing control operation that is not affected by a change in a hydraulic pressure of an internal combustion engine. For example, Japanese Unexamined Patent Publication No. 2006-70754 (corresponding to US2006/0042579A1) discloses one such electrically driven variable valve timing apparatus. The variable valve timing apparatus includes a first gear (an outer gear), a second gear (an inner gear), a phase variable gear (a planetary gear) and an electric motor. The first gear is coaxial with the camshaft of the internal combustion engine and is rotated by a rotational drive force of a crankshaft. The second gear is rotated integrally with the camshaft. The phase variable gear revolves along a circular path, which is coaxial with the camshaft. The phase variable gear transmits a rotational force of the first gear to the second gear and changes a rotational phase of the second gear relative to the first gear. The motor is placed coaxially with the camshaft to control a revolving speed of the phase variable gear. The number of teeth of the first gear and the number of teeth of the second gear are set to drive the camshaft at a corresponding rotational speed, which is one half of the rotational speed of the crankshaft. When the valve timing is not changed, the rotational speed of the motor is adjusted to coincide with the rotational speed of the first gear, which is driven by the crankshaft, and the revolving speed of the phase variable gear is adjusted to coincide with the rotational speed of the first gear. Thereby, the current rotational phase difference between the first gear and the second gear is maintained to maintain the current valve timing. In contrast, at the time of changing the valve timing, the rotational speed of the motor is changed relative to the rotational speed of the first gear to change the revolving speed of the phase variable gear relative to the rotational speed of the first gear. Thereby, the rotational phase difference between the first gear and the second gear is changed to change the valve timing.
However, even when the electric power is supplied to the motor during the non-operational state (stop state) of the internal combustion engine, it is difficult to change the actual camshaft phase (the actual valve timing). Therefore, in this state, when the supply of the electric power to the motor is maintained until the time of coinciding the actual camshaft phase with a target camshaft phase, the electric current is kept flowing only through the respective winding of the same phase in the substantially locked state of the motor. As a result, the temperature of the windings of the motor may possibly exceed an allowable temperature range. This may lead to deterioration of the durability of the motor and/or a failure of the motor.